Recently, I learned that there is at least one Spanish word that can be pronounced but not written.
It is the imperative form of 'salirle'. It is prononunced as 'sal-le' and the written form should ...

In the question "Are there any words that have opposite regional meanings?" there is a list of Spanish words each one having two opposite meanings. Is there a name (in Spanish) for this kind of words?
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In the novel Los años con Laura Diaz by top Mexican author Carlos Fuentes there is a word, "barrocanrolera", which is not in the DRAE, the Gran diccionario Larousse, Wiktionary, or Google Translate.
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